The only thing I worry about Incredibles 2 and sequels in genral is not having the original cast in the movie.
There's really nothing to worry about, Pixar has a great track record when it comes to bringing back the original voices. Monsters University, Cars 2, Finding Dory, the Toy Story sequels have all brought back the voices from the originals. Even in Toy Story 3, they could've gotten any random voice actor to play the older versions of Andy and Sid, Sid didn't even have a very big role in the film, but they still stuck with the same actors who played them as children.